This invention pertains to the art of lift assemblies and more particularly to lift assemblies for elevating automotive vehicles above the ground surface for servicing and repair.
The invention is particularly applicable to a four bar linkage lift assembly and will be described with particular reference thereto. However, it will be appreciated that certain aspects of the invention have broader applications and may be advantageously employed in other environments and applications.
Prior lift devices oftentimes employ two pairs of swing arms interconnecting a base member and a lift platform. The swing arms are disposed on opposite sides of the base member and lift platform, and pivotally secured thereto. One or more hydraulic cylinders elevate the lift platform to selected heights above the base member to permit a mechanic to work on a vehicle raised by the platform. This arrangement is commonly known as a four bar linkage lift assembly.
Typically a stiff leg extends between the base member and lift platform to provide a back-up or safety locking arrangement should any problems arise with the hydraulic cylinder once the platform has been raised. One end of the stiff leg is pivotally secured to the lift platform while the opposite end is adapted for selective sliding movement along the base member. Raised stops or locking members are provided on the base member to cooperate with the stiff leg and prevent inadvertent descent of the lift member relative to the base member.
According to one lift device of this type, the lift platform is a generally rectangular member that is adapted to engage the vehicle at an area between the front and rear axles. For example, a series of rubber pads or "bricks" are positioned on the platform to define the points of contact with the underside of the vehicle. The higher, though, the platform is elevated, the longer the swing arms must be to provide this increased height. In turn, the rear wheels of certain vehicles will therefore have a tendency to impinge on the rear swing arms that extend axially beyond the platform. The problem with such an arrangement is that as the platform is raised, the vehicle may have a tendency to move or roll because of the contact between the rear wheels and the rear swing arms.
Shortening the axial length of the platform encounters the problem that the center of gravity of large wheel base vehicles could extend beyond the surface area of the platform if the operator is not careful in positioning the vehicle over the platform. Thus, there is a tradeoff between the size of the platform, the goal of raising the platform to a predetermined height, the length of the swing arms, and the ability to accommodate as many different size vehicles as possible.
Another characteristic of lift assemblies of this type is that preferably the two front swing arms are folded or stored beneath the lift platform when the platform is in a lowered or down position. This is useful from the standpoint that those swing arms are protected by the lift platform in a lowered position. On the other hand, the height of the lift assembly in its lowered position must be minimized so that it does not interfere with the undercarriage of an automotive vehicle that is driven over the lift assembly to prepare the vehicle for lifting. Minimizing the height, though, restricts the initial lifting ability of the hydraulic cylinder since it must operate through extremely small angles. Thus, a difficult tradeoff exists between the storage of the swing arms, the height requirements, and the lift capabilities of the design.
Another lift device of this general type uses an elongated member that fits laterally between the wheels of the vehicle. The member is sized to permit the wheels of an associated vehicle to straddle either side of the elongated member and the wheel base dimension between the front and rear wheels is not the limiting factor. Dimensional tradeoffs, though, exist with this type of lift device. Particularly, the elongated member is sensitive to the lateral dimension between the insides of the tires.
It is still desirable, though, to engage the underside of a vehicle at selected points with this type of lift device. That is, the undercarriage is not directly engaged but rather it has been deemed desirable to provide an adjustable frame engaging member so that different frame points on various vehicles can be universally accepted.
Another problem associated with lift assemblies of these types is associated with changing the elevated height of a vehicle. For example, if the vehicle is elevated approximately 30 inches above the ground surface and it is desired to lower the vehicle to an intermediate height, currently available lift assemblies require that the platform be completely lowered before it is elevated again to an intermediate height. Of course, this is cumbersome for the lift operator o mechanic to follow this process.
It has thus been deemed desirable to provide a lift assembly that overcomes these various problems and provides a suitable tradeoff between these seemingly incompatible requirements.